Every woman is different, one trick I've found is use the physical and vocal cues she gives you, especially during oral sex. Also use communication prior, and during, tell her to "guide" you. Let her gently grab your head, play with your hair, or whatever, and that will be a very effective tool in gauging orgasm (in a lot of women, not ALL of them). Don't stay in one spot, move around, try different movements of your tongue (or fingers if you are using those as well). Don't just go for what you know to be common erogenous zones like the clit, work around it, and some women respond to different types of stimulation, such as squeezing the entire skin area around the clit as opposed to direct contact, or prefer a specific motion, like flicking as opposed to vibrating, just try different things, and her hands will give you good indication, it can be difficult at times due to tongue cramps and the need to breathe, but pay attention to when she seems in a "heightened" sense, where she starts becoming more vocal, her breathing changes, her body tenses or stiffens, her grip on you becomes tighter.... if any of that occurs keep doing what you're doing, at least for a bit longer, if her reactions get more intense, don't stop, and focus yourself on what you are doing, if you focus it you can get her to orgasm, however if she only momentarily reacts and then regresses back to her normal state, keep trying variations. In some women persistent attention to a certain spot will trigger the release, in others it's a pattern that you have to find, and consistent stimulation will make the area over sensitive to the point where the effectiveness of it wears off.
Oh, and after she cums if she pushes your head away, don't be offended, it's probably just because you made her too sensitive to be touched directly after orgasm.